1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus. More particularly, the present invention relates to a structure for folding a sheet stack for bookbinding.
2. Description of the Related Art
Up to now, there are image forming apparatuses represented by a digital copying machine, and provided with scanning function for reading an image from a document and printing function for printing the scanned image on a sheet such as recording paper. Some of such the image forming apparatuses include a sheet processing apparatus for, after an image is formed, taking in a delivered sheet, stitching the sheet substantially around a center line of the sheet, folding the sheet or the like, and then delivering and stacking the sheet on a sheet stacking portion.
Such the sheet processing apparatuses include a saddle stitch bookbinding apparatus which saves space, is small, and is low-priced. In such the saddle stitch bookbinding apparatus, after stacking and aligning sheets which are transported one by one in a substantially perpendicular vertical intermediate stacking tray, a stapler saddle stitches the sheet at a center portion. After that, an abut plate and a folding roller fold the sheet at the center portion to conduct bookbinding.
Such the conventional saddle stitch bookbinding apparatus is provided with a leading edge regulating member for, when a sheet is brought in the intermediate stacking tray, aligning a sheet by being brought into contact with a leading edge of the sheet. It is to be noted that the leading edge regulating member is set to be in such a position as to stitch the sheet at the center portion in a direction of its transportation and in such a position as to fold the sheet at a stitch position according to the size of the sheet which is brought in.
It is to be noted that, some of such the apparatuses are structured such that, when the stitch position and the fold position are misaligned with the center portion due to variation in the size of the sheet, a user can adjust the height of the sheet when stitched and folded (see Japanese Patent Application Laid-open No. 2001-206626).
In such the conventional sheet processing apparatus (saddle stitch bookbinding apparatus), a fold process is conducted always by abutting the center portion of the sheet against the abut plate, irrespective of the size and thickness of the sheet and the number of sheets in the stack of the sheet. However, when the center portion of the sheet is abutted against the abut plate in this way, and in particular, when the intermediate stacking tray is disposed substantially perpendicularly, there is a problem that the fold position varies depending on a self weight and a solidity of the sheet and the fold position of the sheet is misaligned.
Next, a mechanism where the fold position is misaligned in this way is described with reference to FIG. 8.
FIG. 8A illustrates a state just before abutting and folding operation after a sheet S is brought in an intermediate stacking tray 412 disposed substantially perpendicularly, aligned and saddle stitched, and is then transported in a stack to a half-fold position where the lower edge of the sheet is brought into contact with a leading edge regulating member 411.
In this state, when a sheet stack Sa is abutted against an abut plate 418, in the case of the perpendicular intermediate stacking tray 412, the structure is constructed such that a lower edge of the sheet stack Sa is lifted by friction force between the abut plate 418 and the sheet S until a folded portion of the sheet stack Sa enters a nip between folding rollers 413.
FIG. 8B illustrates a state where the abutted sheet stack Sa consists of a small number of, for example, two sheets. By abutting a center portion of the sheet stack Sa in a direction of transportation against a leading edge of the abut plate 418, the lower edge of the sheet stack Sa is lifted from the leading edge regulating member 411. In this state, the folded portion of the sheet stack Sa is introduced into the nip between the folding rollers 413.
FIG. 8C illustrates a state where the abutted sheet stack Sa consists of a large number of, for example, twenty sheets. In this case, due to the self weight of the sheet stack Sa, it is not possible to make the sheet stack Sa lifted and enter the nip between the folding rollers 413 by the abut plate 418 alone. In this case, the result is that the sheet stack Sa is folded at a position higher than a center position of the sheet stack Sa.
As illustrated in FIG. 8D, when the shape of a path in the intermediate stacking tray 412 in a position higher than the fold position is curved in terms of the structure of the apparatus, contrarily to the above-mentioned case, due to a transportation resistance in the curved path, the sheet S can not go down when the sheet S is abutted against the abut plate 418. As a result, the sheet stack Sa is folded at a position lower than the center position of the sheet stack Sa.
It is to be noted that, since the influence of the curved path varies depending on body of the sheet S, i.e., the thickness and the size of the sheet S, the amount of misalignment depends on the thickness and the size of the sheet. Further, the misaligned fold position depending on the number of sheets in the sheet stack and the size and the thickness of the sheet is also influenced by coefficient of friction between the leading edge of the abut plate 418 (which is brought into contact with the sheet) and the sheet S, and thus, the surface state of the sheet (in other words, whether it has an image formed thereon or not) is also a factor of the misaligned fold position.
FIG. 9 illustrates a state of a brochure B with the misaligned fold position caused by the above-mentioned factors. Even if the saddle stitch position is in a normal position, if the fold position is misaligned by a distance L, misalignment of 2 L is caused at a front edge of the brochure B and degrades an appearance of the brochure B. Conventionally, when misalignment is caused in the formed brochure B, a user measures the distance L and adjusts the saddle stitch position or the half-fold position. However, this includes waste of sheets. Further, such adjustment is necessary with regard to each kind of jobs, and is troublesome.